Gear Oil Compositions, Methods of Making and Using Thereof

ABSTRACT

A gear oil composition is provided. The composition comprises a synergistic blend of mineral oil base stock and polyalphaolefin (PAO) base stock for the oil composition to have a traction coefficient at a slide to roll ratio of 40 percent at 15 mm 2 /s. of 0.030 or less and a pressure viscosity coefficient of at least 16.0 at 80° C., 20 Newton load, and 1.1 m/s rolling speed. In one embodiment, the synergistic amount of PAO base stock ranges from 5 to 48 wt. % based on the total weight of the gear oil composition.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 11/863,068 filed Sep. 27, 2007, the disclosure of which isincorporated herein by reference.

TECHNICAL FIELD

The invention relates generally to compositions suitable for use aslubricants, more particularly for use as gear oils.

BACKGROUND

Gear oil is used in industrial applications as well moving equipmentsuch as automobiles, tractors, and the like (collectively referred to as“equipment”). When in use in some applications, the gear oil is presentas an oil film between the moving parts, e.g., traction drives. Intraction drive applications, power is transmitted via the gear oil film.In some applications, e.g., a hypoid gear of final reduction gear, it isvery desirable to form/retain a thick oil film between gears. Increasedoil film thickness to a sufficient level can protect a friction surfacefrom damages, greatly improving gear and/or bearing fatigue life andload resistance characteristics.

Traction coefficient is the force required to move a load, divided bythe load. The coefficient number expresses the ease with which thelubricant film is sheared. It is desirable for gear oils to have a lowtraction coefficient as the lower the traction coefficient, the lessenergy is dissipated due to lubricant shearing.

Besides having a low traction coefficient, it is important for a gearoil to have a high pressure-viscosity coefficient. Thepressure-viscosity coefficient (“PVC”) refers to the relationshipbetween the load placed on the oil film (pressure) at the dynamic loadzone and the thickness of the oil film (viscosity) at that load, whenall other factors (material, temperature, geometry, speed, load) areconstant. The pressure-viscosity coefficient of a gear oil is a fixedvalue for an oil film thickness in a given set of conditions(elastohydrodynamic regime, also known as an EHL or EHD regime) based ona mathematical estimation as noted in the American Gear ManufacturersAssociation (AGMA) Information Sheet AGMA 925-A03. It is desirable forgear oils to have a high PVC value.

US Patent Publication No. 2007/0027042 discloses a gear oil compositioncomprising two mineral base oil and/or hydrocarbon-based synthetic oilsof different kinematic viscosities, one of 3.5 to 7 mm²/s at 100° C. andone of 20-52 mm²/s at 100° C. US Patent Publication No. 2007/0078070discloses a gear oil composition comprising at least one Group II basestock and at least one low volatility low viscosity polyalphaolefin basestock.

There is still a need for gear oil compositions having a low tractioncoefficient, a high pressure-viscosity coefficient, and optimal filmthickness properties.

SUMMARY OF THE INVENTION

In one aspect, the invention relates to a gear oil compositioncomprising: a) a base oil containing a synergistic mixture of at least apolyalphaolefin base stock with a mineral oil base stock having akinematic viscosity of 3 to 120 mm²/s at 100° C. and a viscosity indexof at least 60; b) 0.001 to 30 wt % at least an additive selected fromtraction reducers, dispersants, viscosity modifiers, pour pointdepressants, antifoaming agents, antioxidants, rust inhibitors, metalpassivators, extreme pressure agents, friction modifiers, and mixturesthereof; wherein the polyalphaolefin is present in a synergistic amountfor the gear oil composition to have a traction coefficient at 15 mm²/sof 0.030 at a slide to roll ratio of 40 percent or less and a pressureviscosity coefficient of at least 16.0 GPa⁻¹ at 80° C., 20 Newton load,and 1.1 m/s rolling speed.

In another aspect, the invention relates to a method for improving thetraction coefficient property of a gear oil, the method comprises addingto a base oil typically used for preparing the gear oil a synergisticamount of at least a polyalphaolefin for the gear oil to have a tractioncoefficient at 15 mm²/s. of 0.030 or less. In one embodiment, thesufficient amount of the polyalphaolefin to be added to the base oilmatrix ranges from 5 to 48 wt. % based on the total weight of the gearoil composition.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph comparing the pressure-viscosity coefficients of thegear compositions of Examples 1-5 at different temperatures.

FIG. 2 is a graph comparing the film thickness of the gear compositionsof Examples 1-5 at different temperatures.

DETAILED DESCRIPTION

The following terms will be used throughout the specification and willhave the following meanings unless otherwise indicated.

“Kinematic viscosity” is a measurement in mm²/s of the resistance toflow of a fluid under gravity, determined by ASTM D445-06.

“Viscosity index” (VI) is an empirical, unit-less number indicating theeffect of temperature change on the kinematic viscosity of the oil. Thehigher the VI of an oil, the lower its tendency to change viscosity withtemperature. Viscosity index is measured according to ASTM D 2270-04.

Cold-cranking simulator apparent viscosity (CCS VIS) is a measurement inmillipascal seconds, mPa·s to measure the viscometric properties oflubricating base oils under low temperature and high shear. CCS VIS isdetermined by ASTM D 5293-04.

The boiling range distribution of base oil, by wt %, is determined bysimulated distillation (SIMDIS) according to ASTM D 6352-04, “BoilingRange Distribution of Petroleum Distillates in Boiling Range from 174 to700° C. by Gas Chromatography.”

“Noack volatility” is defined as the mass of oil, expressed in weight %,which is lost when the oil is heated at 250° C. with a constant flow ofair drawn through it for 60 min., measured according to ASTM D5800-05,Procedure B.

Brookfield viscosity is used to determine the internal fluid-friction ofa lubricant during cold temperature operation, which can be measured byASTM D 2983-04.

“Pour point” is a measurement of the temperature at which a sample ofbase oil will begin to flow under certain carefully controlledconditions, which can be determined as described in ASTM D 5950-02.

“Auto ignition temperature” is the temperature at which a fluid willignite spontaneously in contact with air, which can be determinedaccording to ASTM 659-78.

“Traction coefficient” is an indicator of intrinsic lubricantproperties, expressed as the dimensionless ratio of the friction force Fand the normal force N, where friction is the mechanical force whichresists movement or hinders movement between sliding or rollingsurfaces. Traction coefficient can be measured with an MTM TractionMeasurement System from PCS Instruments, Ltd., configured with apolished 19 mm diameter ball (SAE AISI 52100 steel) angled at 220 to aflat 46 mm diameter polished disk (SAE AISI 52100 steel). The steel balland disk are independently measured at an average rolling speed of 3meters per second, a slide to roll ratio of 40 percent, and a load of 20Newtons. The roll ratio is defined as the difference in sliding speedbetween the ball and disk divided by the mean speed of the ball anddisk, i.e. roll ratio=(Speed1−Speed2)/((Speed1+Speed2)/2).

Molecular weights are determined by ASTM D2503-92 (Reapproved 2002). Themethod uses thermoelectric measurement of vapour pressure (VPO). Incircumstances where there is insufficient sample volume, an alternativemethod of ASTM D2502-04 may be used; and where this has been used it isindicated.

Density is determined by ASTM D4052-96 (Reapproved 2002). The sample isintroduced into an oscillating sample tube and the change in oscillatingfrequency caused by the change in the mass of the tube is used inconjunction with calibration data to determine the density of thesample.

Component A—Group V Polyalphaolefins (“PAOs”): Component A of the baseoil matrix is a Group IV base oil or a mixture of different Group IVbase oils. Group IV base stocks consist of polyalphaolefins (“PAOs”),offering superior volatility, thermal stability, oxidative stability andpour point characteristics compared to those of the Group II and IIIoils with less reliance on additives.

PAOs comprise a class of hydrocarbons manufactured by the catalyticoligomerization (polymerization to low-molecular-weight products) oflinear α-olefins typically ranging from 1-octene to 1-dodecene, althoughpolymers of lower olefins such as ethylene and propylene can also beused, including copolymers of ethylene with higher olefins. Highviscosity PAOs may be conveniently made by the polymerization of anα-olefin in the presence of a polymerization catalyst such as theFriedel-Crafts catalysts including, for example, aluminum trichloride,boron trifluoride or complexes of boron trifluoride with water, alcoholssuch as ethanol, propanol or butanol, carboxylic acids or esters such asethyl acetate or ethyl propionate.

In one embodiment, the PAO used is predominantly α-olefin, that is,linear terminal olefin. By predominantly is meant that the PAO containsover about 50 mole percent of α-olefins. In another embodiment, the PAOis a high viscosity PAO, comprising hydrogenated polymers or oligomersof α-olefins. The α-olefins include, but are not limited to, C₂ to aboutC₃₂ α-olefins, e.g., 1-octene, 1-decene, 1-dodecene and the like. In oneexample, the PAO is a α-olefins selected from the group ofpoly-1-octene, poly-1-decene, and poly-1-dodecene.

The PAO products for use in the composition can have a wide range ofviscosities, varying from highly mobile fluids of low-viscosity, about 2mm²/s., at 100° C. to higher molecular weight, viscous materials whichhave viscosities exceeding 1000 mm²/s (cSt.) at 100° C. In oneembodiment, the PAO products have a viscosity ranging from 40 to 500mm²/s (cSt.) at 40° C. In one embodiment, the PAO for use as component Ahas a viscosity of greater than or equal to about 80 mm²/s at 40° C. andless than or equal to about 20 mm²/s at 100° C. In another embodiment,the PAO base stock has a kinematic viscosity 440° C. in the range of80-110 mm²/s. and a kinematic viscosity @100° C. of 10-16 mm²/s. and aviscosity index of 140-160. In yet another embodiment, the PAO basestock is a blend of different PAOs, one having a viscosity of rangingfrom 30-60 mm²/s at 40° C. and the other having a viscosity of 300-600mm²/s at 40° C., for a PAO blend having a viscosity of 100 mm²/s at 40°C.

Component B—Mineral Oil: Component B is a mineral oil or mixtures ofmineral oils. The mineral oil can be any of paraffinic and naphthenicoils, or mixtures thereof. Mineral oils can be obtained by subjecting alubricating oil fraction produced by atmospheric- or vacuum-distilling acrude oil, to one or more refining processes such as solventdeasphalting, solvent extraction, hydrocracking, solvent dewaxing,catalytic dewaxing, hydrorefining, sulfuric acid treating, and claytreatment.

In one embodiment, the mineral oil used as Component B may contain anamount of synthetic oils such as poly-α-olefins, ethylene-α-olefinscopolymer, and ester-based synthetic oils, in an amount of 50 wt. % orless of the total weight of the gear oil composition.

In one embodiment, Component B is a mineral oil (or blends of mineraloils and/or hydrocarbon-based synthetic oils) having a kinematicviscosity of 3 to 120 mm²/s at 100° C. and a viscosity index of at least60. In another embodiment, Component B is a mineral oil having akinematic viscosity of 2.3 to 3.4 mm²/s at 100° C. and a % Cp defined byASTM D 3238 (R2000) is 70 or higher, ASTM D 3238 is a standard testmethod for calculation of Carbon distribution and structural groupanalysis of petroleum oils by the ndM method. In yet another embodiment,Component B is a base oil matrix having a kinematic viscosity of lessthan 80 mm²/s at 40° C., comprising a mixture of: a “low viscosity”mineral or and/or a synthetic oil having and a kinematic viscosity of3.5 to 7 mm²/s at 100° C.; and a “high viscosity” mineral-based oiland/or hydrocarbon-based synthetic oil having a kinematic viscosity of20 to 52 mm²/s at 100° C.

In one embodiment, the base oil matrix contains sufficient amounts ofmineral and PAO oils for the base oil matrix to have a kinematicviscosity at 100° C. between 10 mm²/s and 15 mm²/s; a kinematicviscosity at 40° C. between 95 mm²/s and 110 mm²/s; and a viscosityindex between 95 and 175.

Additional Optional Components: The incorporation of synergistic amountsof mineral and PAO oils allows the composition to have a low tractioncoefficient without the need for traction reducers in the prior art.However, in one embodiment, small amounts of traction reducers, e.g.,from 0.5 to 10 wt. %, can be incorporated in the gear oil composition.Examples of traction reducers include ExxonMobil's Norpar™ fluids(comprising normal paraffins), Isopar™ fluids (comprising isoparaffins),Exxsol™ fluids (comprising dearomatized hydrocarbon fluids), Varsol™fluids (comprising aliphatic hydrocarbon fluids), and mixtures thereof.

In one embodiment, the gear oil composition comprises 0.01 to 30 wt. %of one or more additives selected from dispersants, viscosity indeximprovers, pour point depressants, antifoaming agents, antioxidants,rust inhibitors, metal passivators, extreme pressure agents, frictionmodifiers, etc., in order to satisfy diversified characteristics, e.g.,those related to friction, oxidation stability, cleanness and defoaming,etc.

Examples of dispersants include those based on polybutenyl succinic acidimide, polybutenyl succinic acid amide, benzylamine, succinic acidester, succinic acid ester-amide and a boron derivative thereof. Whenused, ashless dispersants are typically employed in an amount of 0.05 to7 wt. %. In one embodiment, the dispersant are selected from theproducts of reaction of a polyethylene polyamine, e.g. triethylenetetraamine pentaamine, with a hydrocarbon-substituted anhydride made bythe reaction of a polyolefin, having a molecular weight of about700-1400 with an unsaturated polycarboxylic acid or anhydride, e.g.maleic anhydride.

Examples of metallic detergent include those containing a sulfonate,phenate, salicylate of calcium, magnesium, barium or the like. Metallicdetergents when used, are typically incorporated in an amount of 0.05 to5 wt. %.

Examples of antioxidants include but are not limited to amine-basedones, e.g., alkylated diphenylamine, phenyl-α-naphtylamine and alkylatedphenyl-x-naphtylamine; phenol-based ones, e.g., 2,6-di-t-butyl phenol,4,4′-methylenebis-(2,6-di-t-butyl phenol) andisooctyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate; sulfur-basedones, e.g., dilauryl-3,3′-thiodipropionate; and zinc dithiophosphate.When used, antioxidants are incorporated in an amount from 0.05 to 5 wt.%.

Defoaming agents can be optionally incorporated in an amount of 10-100ppm. Examples of defoaming agents include but are not limited todimethyl polysiloxane, polyacrylate and a fluorine derivative thereof,and perfluoropolyether. Rust inhibitors can be used in an amount from 0to 30 wt. %. Examples include a fatty acid, alkenylsuccinic acid halfester, fatty acid soap, alkylsulfonate, polyhydric alcohol/fatty acidester, fatty acid amine, oxidized paraffin and alkylpolyoxyethyleneether.

Friction modifiers can be incorporated in an amount from 0.05 to 5 wt.%. Examples include but are not limited to organomolybdenum-basedcompounds, fatty acids, higher alcohols, fatty acid esters, sulfidedesters, phosphoric acid ester, acid phosphoric acid esters, acidphosphorous acid esters and amine salt of phosphoric acid ester.

Anti-wear and/or extreme pressure agents can be incorporated in anamount from 0.1 to 10 wt. %. Examples of anti-wear and/or extremepressure agents include metal-free sulfur containing species includingsulfurized olefins, dialkyl polysulfides, diarylpolysulfides, sulfurizedfats and oils, sulfurized fatty acid esters, trithiones, sulfurizedoligomers of C2-C8 monoolefins, thiophosphoric acid compounds,sulfurized terpenes, thiocarbamate compounds, thiocarbonate compounds,sulfoxides, thiol sulfinates, and the like. Other examples includemetal-free phosphorus-containing antiwear and/or extreme pressureadditives such as esters of phosphorus acids, amine salts of phosphorusacids and phosphorus acid-esters, and partial and total thio analogs ofthe foregoing. In one embodiment, the composition comprises an acidphosphate as an anti-wear agent, with the agent having the formulaR₁O(R₂O)P(O)OH, where R₁ is hydrogen or hydrocarbyl and R₂ ishydrocarbyl.

Pour point depressant can be incorporated in an amount ranging from 0.05to 10 wt. %. Examples include but are not limited to ethylene/vinylacetate copolymer, condensate of chlorinated paraffin and naphthalene,condensate of chlorinated paraffin and phenol, polymethacrylate,polyalkyl styrene, chlorinated wax-naphthalene condensate, vinylacetate-fumarate ester copolymer, and the like.

In one embodiment, the composition further comprises at least one of apolyoxyalkylene glycol, polyoxyalkylene glycol ether, and an ester as asolubilizing agent in an amount from 10 to 25 wt. %. Examples includeesters of a dibasic acid (e.g., phthalic, succinic, alkylsuccinic,alkenylsuccinic, maleic, azelaic, suberic, sebacic, fumaric or adipicacid, or linolic acid dimmer) and alcohol (e.g., butyl, hexyl,2-ethylhexyl, dodecyl alcohol, ethylene glycol, diethylene glycolmonoether or propylene glycol); and esters of a monocarboxylic acid of 5to 18 carbon atoms and polyol (e.g., neopentyl glycol,trimethylolpropane, pentaerythritol, dipentaerythritol ortripentaerythritol); polyoxyalkylene glycol ester; and phosphate ester.

In one embodiment, the composition further comprises at least a metalpassivator, and sometimes specifically a copper passivator. Examplesinclude thiazoles, triazoles, and thiadizoles. Specific examples of thethiazoles and thiadiazoles include 2-mercapto-1,3,4-thiadiazole,2-mercapto-5-hydrocarbylthio-1,3,4-thiadiazoles,2-mercapto-5-hydrocarbyldithio-1,3,4-thiadiazoles,2,5-bis-(hydrocarbylthio)-1,3,4-thiadiazoles, and2,5-bis-(hydrocarbyldithio)-1,3,4-thiadiazoles. Other suitableinhibitors of copper corrosion include imidazolines, described above,and the like.

In one embodiment, the composition further comprises at least aviscosity modifier in an amount of 0.50 to 10 wt. %. Examples ofviscosity modifiers include but are not limited to the group ofpolymethacrylate type polymers, ethylene-propylene copolymers,styrene-isoprene copolymers, hydrated styrene-isoprene copolymers,polyisobutylene, and mixtures thereof. In one embodiment, the viscositymodifier is a blend of a polymethacrylate having a weight averagemolecular weight of 25,000 to 150,000 and a shear stability index lessthan 5 and a polymethacrylate having a weight average molecular weightof 500,000 to 1,000,000 and a shear stability index of 25 to 60.

The gear oil composition of the invention is characterized has having asynergistic amount of mineral and PAO base stock for the composition tohave a low traction coefficient, a high pressure-viscosity coefficient,and optimal film thickness properties. In one embodiment, thissynergistic amount of PAO base stock ranges from 5 to 48% (based on thetotal weight of the gear oil composition). In a second embodiment, thesynergistic amount of PAO base stock ranges from 15 to 40 wt. %. In athird embodiment, the synergistic amount of PAO base stock ranges from25-30 wt. %. In a fourth embodiment, the synergistic amount of PAO basestock is at least 40 wt. %. In a fifth embodiment, the synergisticamount of PAO base stock ranges from 10 to 35 wt. %.

In one embodiment, the gear oil comprises a blend of 5 to 48 wt. %(based on the total weight of the gear oil composition) of a PAO basestock having a kinematic viscosity at 40° C. of 70-120 mm²/s., akinematic viscosity at 100° C. of 12 to 18 mm²/s., and a viscosity indexof 130-160; and 25-75 wt. % of a group II neutral base oil having akinematic viscosity at 40° C. of 40-120 mm²/s., a kinematic viscosity at100° C. of 8 to 14 mm²/s., and a viscosity index of 80-120.

Properties: In one embodiment, the gear oil composition having asynergistic combination of mineral and isomerized base oils has atraction coefficient at 15 mm²/s. of 0.030 or less, a pressure viscositycoefficient of greater than 15.0 GPa⁻¹ at 80° C., 20 Newton load, and1.1 nm/s rolling speed, and a film thickness of greater than 175 nm at80° C. In another embodiment, the gear oil composition has a filmthickness of at least 160 nm at 90° C. or 130 nm at 100° C. In a thirdembodiment, the gear oil composition has a pressure viscositycoefficient of at least 15.0 GPa⁻¹ at a temperature in the range of70-100° C., 20 Newton load, and 1.1 m/s rolling speed. In a fourthembodiment, the gear oil composition has a traction coefficient at 15mm²/s. of 0.030 or less, at a slide to roll ratio of 40 percent.

In one embodiment for use as an automotive gear oil, the compositionmeets SAE J306 specifications for the designated viscosity grades. Forexample, under the specifications of SAE J-306, the measured viscosityat 100° C. (212° F.) of an SAE 90 gear oil must exceed 13.5 mm²/s after20 hours of testing.

In yet another embodiment, the composition meets at least one ofindustry specifications SAE J2360, API GL-5 and API MT-1, and militaryspecification MIL-PRF-2105E quality level.

Method for Making: Additives used in formulating the gear oilcomposition can be blended into base oil blends individually or invarious sub-combinations. In one embodiment, all of the components areblended concurrently using an additive concentrate (i.e., additives plusa diluent, such as a hydrocarbon solvent). The use of an additiveconcentrate takes advantage of the mutual compatibility afforded by thecombination of ingredients when in the form of an additive concentrate.

In another embodiment, the composition is prepared by mixing the baseoil and the additive(s) at an appropriate temperature, e.g., 60° C.,until homogeneous.

Applications: The composition is useful in any system that includeelements or parts containing gears of any kind and rolling elementbearings. In one embodiment, the composition is used as a gear oil forlubricating industrial gears, e.g., spur and bevel, helical and spiralbevel, hypoid, worm, and the like. In another embodiment, thecomposition is used in automotive/mobile equipment applications andparts, including aircraft propulsion systems, aircraft transmissions,wind turbine gears, automotive drive trains, transmissions, transfercases, and differentials in automobiles, trucks, and other machinery. Inyet another embodiment, the composition is used in wind turbines,plastic extruder gear boxes, and highly loaded gearboxes used inelectricity generating systems, or paper, steel, oil, textile, lumber,cement industries, and the like.

EXAMPLES

The following Examples are given as non-limitative illustrations ofaspects of the invention. Unless specified otherwise, the components inthe examples are as follows (and expressed in wt. % in Table 1):

RLOP is Chevron™ 600R group II heavy neutral oil from ChevronCorporation.

PAO 8 is a highly branched iso-paraffinic polyalphaolefin commerciallyavailable from various sources, including Chevron Phillips as Synfluid™PAO 8 cSt, with a kinematic viscosity at 100° C. of about 7.8, akinematic viscosity at 40° C. of 46.6, a viscosity index of 138, and apour point of −57° C.

PAO 40 is a highly branched iso-paraffinic polyalphaolefin commerciallyavailable from various sources, including Chevron Phillips as Synfluid™PAO 40 cSt, with a kinematic viscosity at 100° C. of about 40, akinematic viscosity at 40° C. of 410, a viscosity index of 145, and apour point of −34° C.

Additive X is an industrial gear sulfurphosphorus containing extremepressure additive commercially available from various sources.

The kinematic viscosity, refractive index, and density are properties ofthe base oil matrix blends, measured using methods known in the art. Thetraction coefficients of the gear oils in the Examples aremeasured/calculated using methods and devices known in the art, e.g., atraction coefficient measurement device disclosed in U.S. Pat. No.6,691,551, or a Twin-Disc machine designed by Santotrac, for measuringin the elastohydrodynamic (EHD) regime under high pressure of at least300,000 psi.

The EHL film thickness is calculated using methods known in the art,e.g., the American Gear Manufacturers Association (AGMA) InformationSheet AGMA 925 equation 65, wherein the EHL film thickness isestablished by the operating temperature of the components. An oil filmthickness is determined by the oil's response to the shape, temperatureand velocity of the surfaces at the contact inlet. The thickness dependsstrongly on entraining velocity and oil viscosity. Thepressure-viscosity coefficient (“PVC”) quantifies the EHLfilm-generating capability of a gear oil, which can be measured by knownmethods. The PVC can be measured either directly by assessing viscosityas a function of pressure using high-pressure apparatus, or indirectlyby measuring film thickness in an optical interferometer. PVC is theslope of the graphs plotting the log of viscosity vs. pressure.

Results of the experiments establish that the synergistic addition of atleast a PAO base stock into the mineral oil helps improve the tractioncoefficient of the gear oil composition, lowering the tractioncoefficient of at least 10% to 0.030 or less at 15° C., with the valuesof 0.028 or below for compositions containing 25 to 75 wt. % of at leasta PAO base stock. The data establishes that the incorporation of asynergistic amount of PAO base stock into a base oil matrix of gear oilcompositions in the prior art, e.g., a base oil matrix containingmineral oil(s), provides a gear oil composition having desired optimalproperties of low traction coefficient (e.g., 0.030 or less) and highpressure viscosity coefficients or PVC (e.g., greater than 15.0 at atemperature of 65° C. or higher—typical temperatures of gearcomponents). In one embodiment, the addition of excessive amounts of PAOmay afford decreased PVC values, with compositions containing 50-75%PAO, for PVC values less than comparable pure PAO or pure mineralcompositions. Hence, an antagonistic effect was observed in these cases.

FIGS. 1 and 2 are graphs comparing the film thicknesses (refractiveindex corrected) and the pressure-viscosity coefficients of the gear oilexamples as a function of temperature. As shown in FIG. 1, a gear oilcomposition consisting essentially of a Group II neutral oil in theprior art shows a relatively moderate PVC profile that exhibits adownward trend toward about 14.0 GPa⁻¹ or less at 100° C. A gear oilcomposition consisting essentially of at least a PAO base stock exhibitslower PVC values than the group II-based oil in the range of 60-70° C.;its PVC value is about 15 GPa⁻¹ or less at less than 70° C., with a PVCvalue of 14.4 GPa⁻¹ at about 60° C. As shown in the examples, combiningthe prior art base oil with synergistic amounts of PAO base stock (e.g.75% RLOP 600R and 25% PAO) results in significantly improved PVC valuesin the broad 60-100° C. range, with a value of greater than 17.6 GPa⁻¹at about 80° C. Also as shown, this composition shows excellent synergywith the PVC values measured at 80° C. and 100° C. being greater thanthe corresponding values of either the PAO base oil-only or groupII-only gear oils. Also as shown, combinations containing excessiveamounts of PAO exhibit the opposite effect. Compositions containing 50%or greater PAO show lower PVC values at 80° C. than the correspondingPVC values for either the PAO base oil-only or prior art group II onlygear oils. The most antagonism is observed for a composition containinga small amount of Group II neutral base oil and a large amount of PAObase stock (i.e. 25% RLOP 600R and 75% PAO). The composition exhibitedantagonistically decreased PVC values in the 60-80° C. range.

TABLE 1 Example 1 Example 2 Example 3 Example 4 Example 5 RLOP Chevron ™600R group II 98.25 73.6875 49.125 24.5625 0 PAO 8 0 15.085 30.17 45.25560.34 PAO 40 0 9.4775 18.955 28.4325 37.91 Additive X 1.75 1.75 1.751.75 1.75 Traction coefficient @15° C. 0.033 0.030 0.026 0.023 0.019Kinematic viscosity @40° C., mm²/s 107 103 100.4 99.59 99.94 Kinematicviscosity @100° C., mm²/s 11.84 12.35 12.8 13.36 14.01 Viscosity Index99 112 123 133 143 Refractive Index 1.4 1.4 1.4 1.4 1.4

For the purpose of this specification and appended claims, unlessotherwise indicated, all numbers expressing quantities, percentages orproportions, and other numerical values used in the specification andclaims, are to be understood as being modified in all instances by theterm “about.” Accordingly, unless indicated to the contrary, thenumerical parameters set forth in the following specification andattached claims are approximations that may vary depending upon thedesired properties sought to be obtained and/or the precision of aninstrument for measuring the value, thus including the standarddeviation of error for the device or method being employed to determinethe value. The use of the term “or” in the claims is used to mean“and/or” unless explicitly indicated to refer to alternatives only orthe alternative are mutually exclusive, although the disclosure supportsa definition that refers to only alternatives and “and/or.” The use ofthe word “a” or “an” when used in conjunction with the term “comprising”in the claims and/or the specification may mean “one,” but it is alsoconsistent with the meaning of “one or more,” “at least one,” and “oneor more than one.” Furthermore, all ranges disclosed herein areinclusive of the endpoints and are independently combinable. In general,unless otherwise indicated, singular elements may be in the plural andvice versa with no loss of generality. As used herein, the term“include” and its grammatical variants are intended to be non-limiting,such that recitation of items in a list is not to the exclusion of otherlike items that can be substituted or added to the listed items.

It is contemplated that any aspect of the invention discussed in thecontext of one embodiment of the invention may be implemented or appliedwith respect to any other embodiment of the invention. Likewise, anycomposition of the invention may be the result or may be used in anymethod or process of the invention. This written description usesexamples to disclose the invention, including the best mode, and also toenable any person skilled in the art to make and use the invention. Thepatentable scope is defined by the claims, and may include otherexamples that occur to those skilled in the art. Such other examples areintended to be within the scope of the claims if they have structuralelements that do not differ from the literal language of the claims, orif they include equivalent structural elements with insubstantialdifferences from the literal languages of the claims. All citationsreferred herein are expressly incorporated herein by reference.

1. A gear oil composition, comprising: a) a base oil comprising amixture of at least a polyalphaolefin (PAO) base stock and a mineralbase stock having a kinematic viscosity of 3 to 120 mm²/s at 100° C. anda viscosity index of at least 60; b) 0.001 to 30 wt % at least anadditive selected from traction reducers, dispersants, viscositymodifiers, pour point depressants, antifoaming agents, antioxidants,rust inhibitors, metal passivators, extreme pressure agents, frictionmodifiers, and mixtures thereof, wherein the polyalphaolefin base stockis present in a synergistic amount for the gear oil composition to havea traction coefficient at 15 mm²/s. of 0.030 or less at a slide to rollratio of 40 percent and a pressure viscosity coefficient of at least15.0 GPa⁻¹ at 80° C., 20 Newton load, and 1.1 μm/s rolling speed.
 2. Thecomposition of claim 1, wherein the PAO base stock is present in asynergistic amount for the gear oil to have a pressure-viscositycoefficient of at least 15.5 GPa⁻¹ in a temperature range of 70-100° C.,20 Newton load, and 1.1 nm/s rolling speed.
 3. The composition of claim1, wherein the PAO base stock is present in an amount ranging from 5 to48 wt. % based on the total weight of the gear oil composition.
 4. Thecomposition of claim 3, wherein the PAO base stock is present in anamount ranging from 15 to 40 wt. % based on the total weight of the gearoil composition.
 5. The composition of claim 4, wherein the PAO basestock is present in an amount of 25 to 35 wt. % based on the totalweight of the gear oil composition.
 6. The composition of claim 5,wherein the gear oil composition has a pressure viscosity coefficient ofat least 16.0 GPa⁻¹ in a temperature range of 70-100° C., 20 Newtonload, and 1.1 μm/s rolling speed.
 7. The composition of claim 1, whereinthe gear oil composition has a traction coefficient at a slide to rollratio of 40 percent at 15 mm²/s. of less than 0.028.
 8. The compositionof claim 7, wherein the gear oil composition has a traction coefficientat a slide to roll ratio of 40 percent at 15 mm²/s. of less than 0.026.9. The composition of claim 1, wherein the gear oil composition has afilm thickness of at least 175 nm at 80° C.
 10. The composition of claim9, wherein the gear oil composition has a film thickness of at least 160nm at 90° C. or at least 130 nm at 100° C.
 11. The composition of claim1, wherein the PAO base stock has a kinematic viscosity at 40° C.ranging from 40-500 mm²/s.
 12. The composition of claim 1, wherein thePAO base stock has kinematic viscosity at 100° C. of 10-16 mm²/s. and aviscosity index of 140-160.
 13. The composition of claim 1, wherein themineral oil has a kinematic viscosity of 2.3 to 3.4 mm²/s at 100° C. anda % Cp defined by ASTM D 3238 (R2000) of 70 or higher.
 14. Thecomposition of claim 1, wherein the mineral oil has a kinematicviscosity of less than 80 mm²/s at 40° C., comprising a mixture of atleast a mineral oil and a synthetic oil having and a kinematic viscosityof 3.5 to 7 mm²/s at 100° C.; and at least a mineral oil and a syntheticoil having a kinematic viscosity of 20 to 52 mm²/s at 100° C.
 15. Thecomposition of claim 1, wherein the mineral oil is a group II neutralbase oil having a kinematic viscosity at 40° C. of 80-120 mm²/s., akinematic viscosity at 100° C. of 10 to 14 mm²/s., and a viscosity indexof 80-120.
 16. A method for improving the traction properties of a gearoil composition, the method comprises adding a synergistic amount of atleast a polyalphaolefin (PAO) base stock to a base oil matrix comprisingat least a mineral oil having a kinematic viscosity of 3 to 120 mm²/s at100° C. and a viscosity index of at least 60, for the gear oilcomposition to have a traction coefficient at a slide to roll ratio of40 percent at 15 mm²/s. of 0.030 or less, a pressure viscositycoefficient of 15.7 or higher at 80° C., 20 Newton load, and 1.1 m/srolling speed, and a film thickness of greater than 175 nm at 80° C.,wherein the PAO base stock has a kinematic viscosity at 40° C. in therange of 80-110 mm²/s. and a kinematic viscosity at 100° C. of 10-16mm²/s. and a viscosity index of 140-160.
 17. The method of claim 16,wherein the synergistic amount of at least a polyalphaolefin (PAO) basestock ranges from 5 to 48 wt. % based on the total weight of the gearoil composition.
 18. The method of claim 17, wherein the synergisticamount of at least a polyalphaolefin (PAO) base stock ranges from 15 to40 wt. % based on the total weight of the gear oil composition.
 19. Themethod of claim 17, wherein the synergistic amount of at least apolyalphaolefin (PAO) base stock ranges from 25 to 35 wt. % based on thetotal weight of the gear oil composition.
 20. The method of claim 16,wherein a synergistic amount of at least a polyalphaolefin (PAO) basestock is added to the base oil matrix for the composition to have apressure viscosity coefficient of at least 15.5 GPa⁻¹ in a temperaturerange of 70-100° C., 20 Newton load, and 1.1 m/s rolling speed.